1. Field of the Invention
The present invention relates to a process for the manufacture of a hollow turbomachine blade in which a plurality of sheets are assembled face to face and diffusion welded together in predetermined areas before being expanded under gas pressure and shaped by superplastic forming. The invention also relates to apparatus for use during a stage of the said process.
The advantages of using large chord blades in turbomachines are particularly evident in the case of fan rotor blades in bypass turbojet engines. However, these blades must cope with severe conditions of use and, in particular, must possess satisfactory mechanical characteristics associated with anti-vibration properties and resistance to impacts by foreign bodies. The need to achieve sufficient speeds at the tips of the blades has furthermore led to research into reducing mass, and this has been achieved, in particular, by the use of hollow blades.
2. Summary of the Prior Art
EP-A-0700738 describes a process for the manufacture of a hollow turbomachine blade, especially a large chord fan rotor blade, the process generally comprising the following steps:
(a) starting from a definition of the blade to be produced, using computer aided design and manufacturing (CAD/CAM) means to create a digital simulation of the flat form of the primary sheet-like parts of the blade; PA1 (b) die-forging the primary parts of the blade in a press; PA1 (c) machining the primary parts; PA1 (d) depositing diffusion barriers on the primary parts according to a predefined pattern which determines the final internal geometry of the blade; PA1 (e) assembling the primary parts and diffusion welding them together under isostatic pressure; PA1 (f) pressurized gas inflation and superplastic shaping of the assembly and, PA1 (g) final machining of the shaped assembly.
FR-A-9511300 additionally proposes that, in view of the compaction of the primary parts in the regions of the diffusion barriers during the welding step, an operation to unstick the primary parts in these regions is carried out in the cold state before commencing the inflation and shaping step (f). This operation is in fact necessary to ensure the proper progress of the inflation and superplastic forming cycle in step (f) while guaranteeing an even supply of neutral gas through the cavities defined by the weldings and the diffusion barrier coatings. This operation of unsticking the parts of the blade between the welded areas permits control of the deformation rates from the very start of the forming cycle in step (f), and avoids the occurrence of excess pressures due to localized adherence of one blade part to another during the inflation cycle, which would lead to an undesirable excessive inflation and a risk of rupture.
The known methods of implementing the unsticking operation are not entirely satisfactory, however, and it is an aim of the invention to provide an improved method of carrying out this operation, and also to provide a device adapted for use in the method.